Plot summary
Two half-brothers, Wirt and Greg, and an unnamed frog are lost in a forest called the Unknown. They meet a woodsman chopping trees, who warns them of "The Beast" who preys on lost wanderers like them. He offers to help and takes them to an old grist mill for shelter. They are followed to the mill by a fearsome creature, which the brothers take to be the Beast. Greg knocks out the Woodsman, and the two brothers attempt to fight "the beast", destroying the mill in the process. "The beast" coughs up a turtle it had swallowed and reverts back to a calm domesticated dog.
Upon waking, the Woodsman is angry that the mill was destroyed. He chastises Wirt that he needs to take responsibility for himself and his younger brother. He warns them that the Beast is still at large, and advises the brothers to seek shelter in a nearby town, and give the frog a proper name.
Credits
Adapted from Wikipedia contributors, Over the Garden Wall. Exact series, overall episode number and English title matched. Reference markers and illustrations removed; paragraph formatting adjusted.